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If we divide U
A
by I
A
we get Z present to the IED at A side.
I
A
+ I
B
U
A
I
A
I
A
Z
A
= = p ·Z
L
+ ·R
f
EQUATION1274-IEC-650 V1 EN-US
(Equation 129)
The infeed factor (I
A
+I
B
)/I
A
can be very high, 10-20 depending on the differences in source
impedances at local and remote end.
Z <
ZL
Z <
E
SA
U
A
U
B
A
B
E
SB
I
A
I
B
R
f
p*ZL
(1-p)*ZL
Z
SA
Z
SB
IEC09000247-1-en.vsd
IEC09000247 V1 EN-US
Figure 124: Influence of fault current infeed from remote line end
The effect of fault current infeed from remote line end is one of the most driving factors for
justify complementary protection to distance protection.
When the line is heavily loaded, the distance protection at the exporting end will have a
tendency to overreach. To handle this phenomenon, the IED has an adaptive built-in algorithm,
which compensates the overreach tendency of zone 1, at the exporting end. No settings are
required for this function.
7.3.2.3
Load encroachment
M17048-589 v8
In some cases the load impedance might enter the zone characteristic without any fault on the
protected line. The phenomenon is called load encroachment and it might occur when an
external fault is cleared and high emergency load is transferred on the protected line. The
effect of load encroachment is illustrated to the left in figure
impedance inside the characteristic is of course not allowed and the way to handle this with
conventional distance protection is to consider this with the settings, that is, to have a
security margin between the distance zone and the minimum load impedance. This has the
drawback that it will reduce the sensitivity of the protection, that is, the ability to detect
resistive faults.
The IED has a built in function which shapes the characteristic according to the right figure of
. The load encroachment algorithm will increase the possibility to detect high fault
resistances, especially for phase-to-earth faults at remote line end. For example, for a given
setting of the load angle
ArgLd for Phase selection with load encroachment, quadrilateral
characteristic function (FDPSPDIS), the resistive blinder for the zone measurement can be
expanded according to the figure
given higher fault resistance coverage without risk for
unwanted operation due to load encroachment. This is valid in both directions.
The use of the load encroachment feature is essential for long heavily loaded lines, where there
might be a conflict between the necessary emergency load transfer and necessary sensitivity
of the distance protection. The function can also preferably be used on heavy loaded medium
long lines. For short lines, the major concern is to get sufficient fault resistance coverage. Load
encroachment is not a major problem. So, for short lines, the load encroachment function
1MRK 505 343-UEN B
Section 7
Impedance protection
201
Application manual
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